This invention relates to glide heads for detecting defects on a disk surface.
Typical magnetic disks comprise an Al substrate, a NiP layer which is plated on the Al, polished and then textured, an underlayer (e.g. Cr or NiP) sputtered on the plated NiP layer, a thin film of magnetic recording material (typically a Co alloy) sputtered on the underlayer, a protective overcoat sputtered on the magnetic film, and a lubrication layer formed on the overcoat. Magnetic disk manufacturing specifications typically require that asperities and depressions on a magnetic disk are smaller than a certain size. Although magnetic disks are typically textured to have a specified roughness, there has been a trend in the industry to make magnetic disks smoother and smoother. Presently, some magnetic disks are specified to have a roughness less than or equal to about 30 .ANG. (3 nm). As the specified roughness is decreased, the size of the asperity or depression that can be tolerated is decreased.
Media certifiers are apparatus for ensuring that the asperities and depressions on a magnetic disk are less than a specified size. Glide heads are used in conjunction with media certifiers to detect the asperities and depressions. Referring to FIG. 1, during use, a disk 10 is rotated in a direction A1, thereby creating an air cushion above the disk. A glide head 12 includes a slider 13 which rests on this air cushion. A piezo-electric transducer 14 is mounted on slider 13. If slider 13 collides with a defect on rotating disk 10, e.g. an asperity 16, the mechanical shock from the collision with asperity 16 causes transducer 14 to create an electrical voltage. Transducer 14 is connected, via wires, to a circuit within the media certifier (not shown). This circuit identifies signals caused by collisions between slider 13 and asperities. The larger the asperity, the larger the electrical voltage created by transducer 14 and sensed by the circuit within the certifier.
There are two types of piezo-electric transducers used in conjunction with prior art glide heads:
a) A "type 1," or "bimorph" transducer, which creates an electrical voltage when the piezo-electric material is bent. PA1 b) A "type 2," or "compression" piezoelectric transducer, which creates an output voltage in response to acoustic waves (or externally applied stresses).
A type 1 transducer provides a stronger output voltage in response to low frequency stresses (e.g. less than or equal to about 100 KHz). A type 2 transducer provides a stronger output at higher frequencies (e.g. as high as 700 KHz). Type 2 transducers are more commonly used in glide heads than type 1 transducers.
Piezo-electric transducers are discussed in "Guide to Modern Piezoelectric Ceramics", published by Morgan Matroc, Inc., Electro Ceramics Division, incorporated herein by reference. Also see Van Vlack, "A Textbook of Materials Technology", published by Addison-Wesley Publishing Co., 1973, pp. 234-238, incorporated herein by reference. As explained in "Guide to Piezoelectric Ceramics," bimorph piezo-electric transducers comprise two layers of piezo-electric material 17, 18 that are bonded together, e.g. as shown in FIG. 2. The bimorph piezo-electric transducer produces an output voltage when it is bent as shown in FIG. 2.
During use, glide head 12 vibrates even when it does not strike a defect. This causes transducer 14 to produce an output voltage noise signal. With use, head 12 tends to become less sensitive to asperities due to wear of slider 13. Thus, the signal to noise ratio ("SNR") tends to decrease with use. Accordingly, it would be desirable to increase the SNR provided by transducer 14.